The objective is to understand the molecular mechanism by which herpes simplex virus type 1 (HSV-1) facilitates cell to cell spread of virus, which governs the development of HSV-induced eye disease, a leading cause of blindness. Primary HSV-1 infection of the eye involves the epithelial cells of the cornea, but subsequently spreads to neurons to establish latency. Latent HSV-1 can reactivate recurrently and travel back to the eye to cause secondary infections. However, mutant HSV that can replicate but not spread beyond the initial site of infection do not induce primary ocular disease in experimental animals, and, lacking the ability to spread to neurons to establish latency, cannot cause recurrent secondary ocular disease. The HSV-1 glycoprotein E/glycoprotein I (gE/gI) complex is critical for efficient spread of virus between cultured epithelial cells and neurons and in vivo. The gE/gI complex accumulates at intercellular junctions, where it may interact with cellular components of intercellular junctions, but the specific molecular mechanism by which gE/gI mediates cell-to-cell spread is unknown. To assess the molecular mechanisms of HSV cell-to-cell spread, we will develop an in vitro single step assay that is rapid, quantitative and amenable to many different experimental parameters. To make correlations between the known characteristics of the gE/gI complex and its function in cell-to-cell spread, we will construct a panel of HSV gE mutant viruses that will be analyzed in vitro and in vivo in a murine model of ocular disease. As a longer term goal, we will begin to identify and characterize cellular ligands of the gE/gI complex, which could prove to be essential for understanding the molecular function of gE/gI in cell-to-cell spread of virus. These studies will provide valuable new information about the important, yet poorly understood, process of cell-to-cell spread and how this process impacts HSV-induced ocular disease.